In speech signal encoding in general, the LSP (Linear Spectral Pairs) parameter is widely used as a parameter for efficiently presenting spectral envelope information. LSP is also referred to as LSF (Linear Spectral Frequency).
LSP parameter (hereinafter simply “LSP”) encoding is an essential elemental technology of speech encoding technology for encoding speech signals at high efficiency, and is an important elemental technology in band scalable speech encoding which hierarchically encodes speech signals to generate narrowband signals and wideband signals associated with the core layer and enhancement layer, respectively, as well.
Patent Document 1 describes one example of a conventional method used to decode encoded LSP obtained from band scalable speech encoding. The scalable decoding method disclosed adds a component decoded in an enhancement layer to 0.5 times the narrowband decoded LSP of the core layer to obtain a wideband decoded LSP.
However, when the above-mentioned encoded LSP is transmitted, a part of the encoded LSP may be lost on the transmission path. When a part of the LSP does not arrive on the decoding side, the decoding side requires a process for concealing the lost information. Thus, in speech communication performed under a system environment where errors may occur during information transmission, use of a loss concealment process is an important elemental technology for improvement of the error resistance of a speech encoding/decoding system. For example, in the loss concealment method described in Patent Document 2, when the LSP of higher seven of ten orders, that were divided prior to transmission into three lower orders and seven higher orders, does not arrive on the decoding side, the LSP of the last successfully decoded seven higher orders is used repeatedly as the decoded value.
Patent Document 1 Japanese Patent Application Laid-Open HEI11-30997
Patent Document 2 Japanese Patent Application Laid-Open HEI9-172413